Manually openable adapter valves are known in the prior art for loading fluid into a transportation tank. Also known are means for holding the poppet of the adapter valve in predetermined open positions. However, such arrangements heretofore have usually been somewhat complex, resulting in increased costs for the adapter valve mechanism. U.S. patent application, Ser. No. 633,629, filed Nov. 20, 1975, in the name of Joseph H. DeFrees and entitled Adapter Valve Mechanism For Transportation Tank, discloses various types of adapter valves.